Enable Slack application on development

Slack application is only meant for GitLab.com. However,
the current conditions make it hard to work on new
features for the app.

This change updates some of the logic to enable the
Slack application when on development.

ee/app/models/ee/integration.rb

@@ -22,7 +22,7 @@ module EE
       override :project_specific_integration_names
       def project_specific_integration_names
         integrations = super + EE_PROJECT_SPECIFIC_INTEGRATION_NAMES
-        integrations += EE_COM_PROJECT_SPECIFIC_INTEGRATION_NAMES if ::Gitlab.com?
+        integrations += EE_COM_PROJECT_SPECIFIC_INTEGRATION_NAMES if ::Gitlab.dev_env_or_com?
         integrations
       end
     end

ee/app/models/ee/project.rb

@@ -658,7 +658,13 @@ module EE
     def disabled_integrations
       strong_memoize(:disabled_integrations) do
         gh = github_integration_enabled? ? [] : %w[github]
-        slack = ::Gitlab::CurrentSettings.slack_app_enabled ? %w[slack_slash_commands] : %w[gitlab_slack_application]
+        slack = if Rails.env.development?
+                  []
+                elsif ::Gitlab::CurrentSettings.slack_app_enabled
+                  %w[slack_slash_commands]
+                else
+                  %w[gitlab_slack_application]
+                end
 
         super + gh + slack
       end